1. Technical Field
The present invention relates to an ultrasonic measurement apparatus, an ultrasonic diagnostic apparatus, an ultrasonic measurement method, and the like.
2. Related Art
In ultrasonic measurement apparatuses (ultrasonic diagnostic apparatuses), synthetic aperture processing in which focusing can be formed in overall region of an observation region is adopted thereto in order to obtain favorable resolution throughout a wide region in an ultrasonic image. According to the aforementioned technique, focusing can be formed with the fewer number of times of transmissions and receptions, and processing is performed so as to be able to attain a high frame rate and high resolution.
Among methods of the synthetic aperture processing, there is known technique in which plane waves are used as transmission waves. For example, JP-A-2003-220059 (Patent Literature 1) discloses technique of performing the synthetic aperture processing in consideration of diffraction characteristics of ultrasonic waves while using the plane waves as transmission waves.
Patent Literature 1 also discloses technique in which improvement of resolution is realized by introducing an adaptive weight with respect to each signal when executing signal synthesis of the synthetic aperture processing. For example, according to Iben Kraglund Holfort et al, “Adaptive Receive and Transmit Apodization for Synthetic Aperture Ultrasound Imaging”, ULTSYM.2009 (Non-Patent Literature 1), improvement of resolution is realized by executing adaptive beamforming with respect to an synthetic aperture technique in which a transmission method using spherical waves is adopted, and performing weighting addition with respect to low-resolution signals obtained through each transmission. Meanwhile, according to Andreas Austeng et al, “Coherent Plane-Wave Compounding and Minimum Variance Beamforming” ULTSYM.2011 (Non-Patent Literature 2) which is applied with Non-Patent Literature 1, improvement of resolution is realized by executing the adaptive beamforming with respect to the synthetic aperture technique in which another transmission method using plane waves is adopted.
In technique disclosed in Non-Patent Literature 2, weights according to a minimum variance beamforming method (hereinafter, referred to as the MVB method) are introduced with respect to low-resolution signals which are obtained through multiple times of transmissions and receptions. However, in the technique, an improvement effect of resolution can be obtained in only a region in which each transmission wave is propagated and superimposed as a plane wave.
The region in which the plane waves are superimposed varies in accordance with a depth (an observation depth) of a signal processing target point with respect to a transmission scanning angle, and when the transmission scanning angle is increased or the observation depth is deepened, there is generation of a signal of a region in which a plane wave propagation model does not come into existence. If the MVB method is executed in such a case, there is an influence caused by a signal other than the plane wave (for example, a spherical wave) so that the improvement effect of resolution cannot be obtained, thereby leading to an occurrence of deterioration of an image. In other words, there is a disadvantage of limitation on a region in which the improvement effect of resolution according to the MVB method can be obtained.